Known liquid printing ink colour mixing methods generally involve colour matching the required shade and presenting the desired shade in terms of a 100 part mixing formula by weight of different base colour inks, dispensing the required amounts of the selected base colour inks to the mixing formula requirements to give the desired total weight of the finished shade ink, and mixing the dispensed base colour inks. Provided that the correct 100 part mixing formula is selected, the basic problem in achieving the desired shade ink is in dispensing and mixing correct quantities of the base colour inks called for by the 100 part mixing formula.
Conventional dispensing techniques and apparatus which dispense the base colour inks by volume or by weight are not sufficiently accurate for mixing printing inks for tinplate printing where the base colour inks may be particularly viscous and vary considerably in specific gravity from one colour to another due to the different weight pigments employed. Additionally such conventional dispensing techniques and apparatus can be quite slow in operation again due to the ink viscosity and need for accurate dispensing of a desired quantity.